1. Field of the Invention
The present invention relates to a cadmium negative electrode for an alkaline storage battery such as a nickel-cadmium storage battery and a manufacturing method of the same.
2. Description of the Prior Art
As a conventional cadmium negative electrode for a nickel-cadmium storage battery, there have been proposed a sintered type negative electrode fabricated by impregnating an active material such as cadmium oxide or cadmium hydroxide into a porous substrate formed by sintering nickel powder and a paste type negative electrode fabricated by the steps of mixing an active material with synthetic fibers and paste, coating the mixture on a conductive substrate in the form of a punched sheet metal and drying the conductive substrate coated with the mixture. In these negative electrodes, the latter paste type negative electrode is widely used since it can be fabricated at a relatively low cost to provide a storage battery of high energy density.
In an alkaline storage battery equipped with the paste type negative electrode, a large amount of active material can be loaded as compared with an alkaline storage battery equipped with the sintered type negative electrode. The utilization rate of the active material in the storage battery is, however, decreased due to lack of a conductive network for collecting an electric current. To solve the problem, proposed in Japanese Patent Laid-open Publication No. 62(1987)-211862 is an alkaline storage battery added with yttrium oxide powder to enhance the utilization rate of the active material.
In the alkaline storage battery disclosed in Japanese Patent Laid-open Publication No. 62(1987)-211862, however, electrical contact between particles of the cadmium active material becomes insufficient due to particles of the yttrium oxide interposed therebetween. When the yttrium oxide is hydrated in alkaline solution and changed to yttrium hydroxide, an increase in volume of the yttrium hydroxide obstructs electric contact of particles of the cadmium active material. This results in deterioration of the utilization rate and cycle characteristic of the cadmium active material.
To solve such a problem, proposed in Japanese Patent Laid-open Publication No. 3(1991)-192655 is an alkaline storage battery added with yttrium hydroxide to enhance the utilization rate of the cadmium active material. In the alkaline storage battery disclosed in Japanese Patent Laid-open Publication No. 3(1991)-192655, the yttrium hydroxide is preliminarily added to the active material powder mainly comprised of the cadmium oxide. Since the yttrium hydroxide does not increase in volume during hydration, sufficient electrical contact between particles of the cadmium active material is obtained without destroying the matrix of particles of the cadmium active material. Furthermore, as compared with the addition of the yttrium oxide, the yttrium hydroxide is uniformly dispersed in the cadmium active material. For these reasons, the utilization rate and cycle characteristic of the active material are enhanced effectively.
In an alkaline storage battery of this kind, the capacity of the negative electrode is determined more than that of the positive electrode so that a dischargeable capacity remains in the negative electrode when the positive electrode has been fully discharged at the end of discharge. Hereinafter, this is referred to a discharge reserve. Since the dischargeable capacity of the negative electrode is influenced by a discharge rate, a discharge temperature or the like, the capacity of the storage battery is sometimes limited by the capacity of the negative electrode. For this reason, the discharge reserve is taken into consideration to effect stable performance of the storage battery in various discharge conditions.
For the discharge reserve, the cadmium negative electrode in the nickel-cadmium storage battery is partly charged to cause partial reduction of the cadmium hydroxide and is assembled with the storage battery after washing and drying. Alternatively, metallic cadmium powder is added to the cadmium hydroxide or cadmium oxide powder so that the metallic cadmium remains as a non-discharged portion in the negative electrode at the end of discharge of the positive electrode. Hereinafter, the metallic cadmium is referred to an active material in charged state.
In the alkaline storage battery disclosed in Japanese Patent Laid-open Publication No. 3(1991)-192655, however, the active material in charged state is not taken into consideration. In general, the utilization rate of the metallic cadmium contained as the active material in charged state is lower than that of metallic cadmium produced by charge of the cadmium oxide or cadmium hydroxide. For this reason, enhancement of the utilization rate of the active material in the negative electrode containing metallic cadmium as the active material in charged state is limited in a certain extent.